There is a class of portable devices that accurately measure three-dimensional coordinates to a range of several tens of meters. Such devices are ordinarily referred to as portable large-scale coordinate-measuring machines (CMMs). One type of portable CMM is the laser tracker. It sends a laser beam to a retroreflector target, which may be a spherically mounted retroreflector (SMR) comprising a cube-corner retroreflector centered within a sphere that is moved over the surface of interest. Alternatively the target may be a retroprobe, which comprises a cube-corner retroreflector positioned near a mirror to form a virtual image of a probe tip (U.S. Pat. No. 5,530,549).
A second category of portable CMM is the camera and multiple-light probe (U.S. Pat. No. 5,440,392). This device comprises a probe, which contains a probe tip and at least three point sources of light, together with one or more cameras that view the point sources of light. The image pattern on the camera is used to determine the position of the probe tip in space.
A third category of portable CMM is the laser tracker with a separate but nearby camera. (U.S. Pat. No. 5,973,788). This device comprises the laser tracker, a camera mounted near the laser tracker, and a probe. The probe comprises a probe tip, at least three point sources of light, and a retroreflector. A laser beam from the laser tracker measures the three-dimensional coordinates of the retroreflector. At the same time, the camera measures the three-dimensional coordinates of the light sources on the probe. The information from these measurements allows the coordinates of the probe-tip in space to be determined. A related idea (U.S. Pat. No. 5,973,788) is to embed the camera into a laser tracker that also contains an absolute distance meter.
Four attributes are desirable in a portable CMM: (1) low price, (2) high accuracy, (3) rapid data collection, and (4) ease-of-use. Today the least expensive of these devices costs nearly $100,000. Some devices collect data too slowly to be used to efficiency determine the coordinates of three-dimensional contours. Other devices have relatively poor accuracy. There is a need today for a new type of instrument that is fast, accurate, and much less expensive than current portable CMMs.
Some advantages of the present three-dimensional coordinate measuring device compared to prior art are discussed herein, however this is not intended to be a limiting or exhaustive listing of advantages. There are several companies that make camera-based metrology systems. One of the highest accuracy Metronor systems uses two cameras, which can be placed on a factory floor for example. In this respect, the Metronor system is like the present device. However, the Metronor system is based on a probe with a tip that is not illuminated. The find the location of this tip, the Metronor system must be capable of accurately determining the pitch, roll, and yaw angles of the probe from the camera images of multiple LEDs located on the probe. To make possible the required accuracy, Metronor makes their probes large enough to cover a relatively large fraction of the field of view of the cameras. Such large probes are cumbersome to manage. Furthermore, the probes need to be stiff and fixed in length over temperature, which means using expensive composite materials.
In most cases a different kind of probe such as the embodiments described below which are not currently used by any camera-based system, would be more useful, convenient, and cost effective than an extended probe such as Metronor's. The simplest such device—the sphere with a light source placed at its center—takes advantage of the fact that the distance from the center to the edge of a sphere is a constant. Thus, the sphere body acts like a spacer of known dimensions between the object to be measured and the light source. Also, the sphere can be turned in any direction. Because of this fact, the spherically mounted light source can be used to measure the contour of almost any object. In some cases, a handle will be also convenient. A similar idea from symmetry leads to the fiducial target, which will typically be put into a pre-established tooling hole. In this case, the light source is located on the rotational axis of symmetry of the target. By measuring the target, it is therefore possible to determine the three-dimensional coordinate of the hole on the tool.
One thing that all of the targets—sphere, probe-mount, fiducial, wide-angle, or retroprobe target—have in common is that all emit light over a wide angle from a single point in space. All of the prior art camera-based probes use multiple points of light. Thus, it has not been obvious to those skilled in the art that such useful and accurate probes could be devised based on a single point-of-light. Proof of this fact is that people have been making camera-based metrology systems for years and that no one has yet implemented such as system, which arguably can be made less expensively and with equal or higher accuracy.